Progressive and tragic case of infiltrative mesenteric desmoid tumor which occurred 5 years following prophylactic total colectomy in a patient with familial adenomatous polyposis (FAP). These patients are at particularly high risk for developing desmoid tumors (especially following surgery), and they are an important cause of morbidity and mortality.

Imaging takeaways:

• while many desmoid tumors are round and well-circumscribed, they can also be infiltrative (or mixed well-defined and infiltrative);
• the degree of soft tissue infiltration should not be mistaken for sclerosis mesenteritis which is typically more central and associated with pseudocapsule/fat ring (early mesenteric panniculitis) or retraction and calcifications (later retractile mesenteritis) 
• while peritoneal lymphoma or mesothelioma can overlap with this appearance, the lack of adenopathy or peritoneal/serosal implants (and the patient history) can rule these out
• have a high suspicion for abdominal tumors or soft tissue when reviewing imaging for FAP patients; while most commonly intra-abdominal, they can also occur at port sites, body wall, and incisions
• notice the progressive narrowing of mesenteric vasculature, consistent with an infiltrative fibrosing process
• tragically, at the end stage this eroded through the bowel wall and resulted in desmoid-enteric fistula and small bowel obstruction